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1. Field of the invention
This invention relates to analytical spectrometry. In particular this invention relates to compact high resolution spectrometry by the simultaneous use of two gratings with either solid-state sensors or photomultiplier tubes.
2. Description of the Related Art
For some time now, the Paschen-Runge setup has been used in analytical spectrometry On the whole it appears due to the fact that diffraction and image formation is accomplished by a single optical elementxe2x80x94a concave grating.
With a given number of grooves per mm and entrance angle a specific spectral section is imaged on the Rowland circle. At the same time the spectral resolution which strongly affects the lower detection limit in spectrometry is determined by the diameter of the Rowland circle and the dispersion of the grating.
The application of large Rowland diameters is in contrast to the demands for a modem compact spectrometer design. Therefor, the spectral resolution needed is preferably achieved by using a large number of grooves per mm. The available spectral range then results from the chosen length of the Rowland circle arc.
Current Paschen-Runge spectrometers using a single diffraction grating retain some drawbacks because of the following facts:
1. Image distortions strongly increase at high diffraction angles deteriorating the spectral resolution. The useable length of the Rowland circle arc and resulting spectral range will thus be reduced.
2. Only a continuous, coherent spectral section can be imaged, so that areas along the Rowland curve may be occupied by wavelength regions which are of no interest for analytical purposes.
The restrictions stated in 1 and 2 often lead to a division of the entire spectral range applying several spectrometer units.
3. Commercial solid-state linear array sensors come in standardized chip carrier packages, the dimensions of which are much larger than the radiation xe2x80x9csensitivexe2x80x9d part of the chips. In order to avoid substantial gaps in the detection of the spectrum the sensors must overlap along the focal curve. This can be done either by inclination of the linear arrays to the plane of dispersion (see Deutsches Patentamt, Offenlegungschrift DE 195 23 140 A1, xe2x80x9cMehrkanal-Spektrometer mit Zeilensensorxe2x80x9d) or by overlapping of horizontally oriented sensor chip carriers. In both cases, the array sensors with regard to the entire detect radiation several millimeters above or below the plane of dispersion. However, in these regions the imaging physics of the Rowland setup causes increased aberrations, resulting in a reduction of the spectral resolution.
Consequently, the features of the invention are;
1. Realization of a single spectrometer unit in which different spectral ranges and corresponding dispersions can be adapted to the requirements of the analytical task.
2. Realization of a detector arrangement along the focal curve through which
(a) only the radiation close to the dispersion plane will be detected thus optimum spectral resolution is guaranteed,
(b) a nearly complete detection of the available spectrum is possible using commercial solid-state linear array sensors.